Final answer:
Skipjack, yellowfin, albacore, bigeye, and bluefin tunas are saltwater species and face threats from overfishing, leading to significant population declines and potential extinction. Option C is correct.
Step-by-step explanation:
Skipjack, yellowfin, albacore, bigeye, and bluefin tunas are most correctly categorized as C) Saltwater species. These species are highly valued in world markets for their meat which is often used in delicacies such as sushi. Overfishing, particularly of the bluefin tuna, has led to significant declines in their populations, with the Atlantic population of bluefins dropping by an estimated 80% due to these practices.
This represents a serious concern as commercial fishing activities disproportionately threaten large-bodied marine and freshwater species, leading to the 'fishing down marine food webs' process and possible extinction of certain marine vertebrates like tunas, sharks, and sea turtles.
The tunas mentioned in the question, including skipjack, yellowfin, albacore, bigeye, and bluefin tunas, are saltwater species. They are typically found in the open ocean and are highly valued in world markets. Overfishing and the illegal trade of these species have contributed to their declining populations in certain regions.